Recently, there has been an increasing interest in energy storage technology. As the application fields of energy storage technologies have been extended to cellular phones, camcorders, notebook computers, PCs and electric cars, efforts have been increasingly been made towards the research and development of electrochemical devices capable of charging and discharging, particular a lithium secondary battery. Also, a recent trend of developing the lithium secondary battery is to design a new electrode and battery capable of improving capacity density and specific energy.
Among currently available secondary batteries, lithium secondary batteries developed in the early 1990's have drawn particular attention due to their advantages of higher operating voltages and much higher energy densities than conventional batteries using an aqueous electrolyte, for example, Ni-MH, Ni—Cd, and H2SO4—Pb batteries. However, such lithium ion batteries suffer from safety problems, such as fire and explosion, when encountered with the use of organic electrolytes and are disadvantageously complicated to fabricate. In attempts to overcome the disadvantages of lithium ion batteries, lithium ion polymer batteries have been developed as next-generation batteries. More research is still urgently needed to improve the relatively low capacities and insufficient low-temperature discharge capacities of lithium ion polymer batteries in comparison with lithium ion batteries.
Many companies have produced a variety of electrochemical devices with different safety characteristics. It is very important to evaluate and ensure the safety of such electrochemical devices. The most important consideration for safety is that operational failure or malfunction of electrochemical devices should not cause injury to users. For this purpose, regulatory guidelines strictly restrict potential dangers (such as fire and smoke emission) of electrochemical devices. Overheating of an electrochemical device may cause thermal runaway or a puncture of a separator may pose an increased risk of explosion. In particular, porous polyolefin substrates commonly used as separators for electrochemical devices undergo severe thermal shrinkage at a temperature of 100° C. or higher in view of their material characteristics and production processes including elongation. This thermal shrinkage behavior may cause a short circuit between a cathode and an anode.
In order to solve the above safety problems of electrochemical devices, a separator comprising a porous coating layer obtained from inorganic particles and a binder polymer has been proposed. Such a separator has been conventionally prepared by coating a slurry of inorganic particles and a binder polymer on the surface of an active material layer which is obtained as a porous membrane applied on a plate, in which the binder polymer is penetrated into the pores of the active material layer to render the active material layer ununiform. In this regard, Korean Patent Application Publication No. 2008-0109237 discloses a method of preparing an electrode by applying a solvent in advance prior to the formation of a porous coating layer to prevent the penetration of a binder polymer. In this case, there is a still problem that the application of the solvent reduces a filling density and a rough surface is formed.